The present invention generally relates to systems and processes for removing acidic gases from gas streams, including but not limited to natural gas, refinery process gas, syngas from gasification, sour water strippers, anaerobic digestion of biomass, and other gases that may contain hydrogen sulfide (H2S). The invention particularly relates to systems and processes for removing hydrogen sulfide from gas streams and oxidizing captured hydrogen sulfide to form thiosulfate, sulfate, or mixture of thiosulfate and sulfate.
Many gas streams, as nonlimiting examples, natural and associated gas, gas streams from biomass processing, coal and oil gasification processes, refinery streams, sour water stripper streams, and many more, contain sulfur in the form of hydrogen sulfide (H2S). Hydrogen sulfide is corrosive, flammable, and explosive, and a hazardous and toxic air pollutant that produces an undesirable odor. Gases containing hydrogen sulfide are known to be hazardous to the environment, toxic, and odorous even at low concentrations of hydrogen sulfide. As a result, the emission of hydrogen sulfide into the atmosphere is closely regulated by clean air statutes, and removal of hydrogen sulfide from gas streams is of importance, particularly if the gas stream or constituents thereof are to be further processed or sold.
Gas liquid scrubbers (also referred to as contactors, absorbers, etc.) are widely employed to remove hydrogen sulfide from gases produced by industrial plants or present in natural gas. Scrubbers generally employ a liquid-containing media, such as amines, glycols, and methanol, which is brought into intimate contact with the gas to remove hydrogen sulfide by absorption. The resulting H2S-rich absorbent is regenerated to produce a concentrated hydrogen sulfide stream that can be either converted to elemental sulfur in a Claus plant (unit) or re-injected into deep wells. Both are complex and very expensive processes.
Numerous physical and chemical processes exist for the capture of hydrogen sulfide from gases. These processes include amine-based chemical processes and physical processes, a notable example of which uses a glycol-based solvent commercially available under SELEXOL®. In these processes, hydrogen sulfide is absorbed into the solvent and then stripped in a concentrated form to become a concentrated hydrogen sulfide gas stream. Most of the streams containing hydrogen sulfide also contain carbon dioxide (CO2), which is captured and stripped together with hydrogen sulfide so that the concentrated stripped gas contains both hydrogen sulfide and carbon dioxide. The presence of carbon dioxide with hydrogen sulfide makes the downstream treatment of an H2S-containing gas stream more complex and expensive.
The concentrated hydrogen sulfide gas stream may be sent to a Claus unit to convert the hydrogen sulfide to elemental sulfur. The Clause process generally oxidizes and converts about one-third of the hydrogen sulfide content of the concentrated gas stream to sulfur dioxide, which then reacts with remaining hydrogen sulfide to produce elemental sulfur (the Claus reaction).2H2S+SO2→3S+2H2OWater vapor, carbon dioxide, residual hydrogen sulfide, and other gas species are emitted from a Claus unit as a tail gas. The tail gas is not allowed to be emitted to the atmosphere, and instead must be treated in a tail gas treatment process to capture the residual sulfur compounds.
An alternative to the Claus process is to oxidize hydrogen sulfide from the concentrated stream to sulfur dioxide by combustion. The sulfur dioxide is then further oxidized to sulfur trioxide (SO3) by a catalytic reaction to produce, by reaction with water, sulfuric acid (H2SO4) as a final product.
Existing processes for hydrogen sulfide capture and treatment have various shortcomings, for example, complex and expensive facilities that require a large foot print, high operating temperatures, energy intensive, and the production of low or negative value products. Consequently, there is an ongoing need for simpler and lower cost processes to capture hydrogen sulfide and convert captured hydrogen sulfide to a valuable product.